Selective antenna circuits



NOV. 7, G. w G|LMAN SELECTIVE ANTENNA CIRCUITS Filed Oct. 19, 1931 7252 M'etel' @teI um i 84 Mi tersf- 22 .75 Zlter Mel/um,

INVENTOR GWQzZIma/a/ BY g ATTORNEY Patented Nov. 7, 1933 UNITED STATES SELECTIVE ANTENNA CIRCUITS George W. Gilman, Lawrenceville, N. .l., assignor to American Telephone and Telegraph Company, a corporation of New York Application October 19, 1931. Serial No. 569,778 9 Claims. (01. 173-44) This invention relates to selective antenna circuits and is a further development of the invention disclosed in my application, Serial No. 480,246, filed September 6, 1930.

The purpose of the invention is to devise an antenna system designed for different frequencies to be so connected that the frequency of the entire station may be changed simply by changing the frequency of the wave used. 19 Another purpose is to, accomplish this without the use of mechanical or electrical switching means calling for any moving parts.

In my application cited above, it was shown that if a transmission line is divided with a branch going to each of a plurality of antennas tuned to different frequencies, it is possible, in transmission, to arrange the line so that energy of a given frequency will flow only down that branch leading to the appropriate antenna. This was accomplished in one case by providing at a certain point in one of the branches, a plurality of networks each of which acted as a short-circuit for one of the waves to be excluded from that branch, thus excluding all but the desired wave.

manner.

In this invention I show how the above results may be attained in a similar manner, especially for the case of two antennas.

The invention will be better understood by reference to the following specification and accompanying drawing, in which Figures 1 to 3 are diagrammatic circuits for explaining the invention, and Fig. 4 shows one mode of application to an antenna system. c

In each of Figs. 1 and 2 there is shown a transmission line divided at the point G into branches A and B leading to antennae operative respectively, at frequencies n and n, or wave lengths x and 40 A. In the branch A it is desired to exclude wave length N. Two conditions arise, one where v is greater than N, and the other'where is less than A. The first is shown in Fig. l where there is connected to, or bridged across the line at a distance from the point of departure C, a short transmission line D of length m and short-circuited at the one end F. The connection of this section D is at a point Each branch was shown as treated in a similar Y wave which is to be excluded. For the wave I,

however, the result is the same as if the bridging circuit were removed, all of which can be shown by the following considerations.

In Fig. 3 there is shown a smooth line of two parallel wires of characteristic impedance Z0 open at one end and terminated in a resistance at the other end, the line being of length 1 and of negligible resistance and leakance. It can be demonstrated by electric circuit theory that if the terminating resistance equals zero the impedance looking towards the short-circuited end and measured. from a point p of distance kl from the short-circuited end is .zppq'zo an e 0 (1) and the impedance looking toward the open' end is tan (1 k)0 I The impedance of the two portions in parallel" as seen from p is Z Z tan ltd where 6=angle of the line;

Here L and C are inductance and capacitance of the line per unit length, w=21r frequency. It

' where Z is the length of the line under consideration. r

It will be noted that if at some frequency, say is, that is it Z is then Equation (3) becomes z. tan 18 1tan k E cot k (4) In other words, for that frequency for which the line is a one-quarterwave length, theimpedance v, offered to a source is infinite, no matter where from the open end, and under these conditions the branch is effectively short-circuited for the the source may be ,connected so long as it is not in the short circuit; that is, so long as k is not equal to zero. On the other hand, Z is equal to zero if tan (1-k) 0: that is, if

where n is any odd integer. Taking the condition where n has the value of unity,"and"in'serti'ng the value of 0, it appears that In other words, if the branch A is honnectedto the transmission line D at a distance rr'din theopen' end or D, thenthe im'pe'danc'e for the 7 frequency corresponding .to the wave, A is equalto zero, and the transmission line A behaves for that frequency as though it were short-circuit- .ed, whereas, for the wave length xfitbehaves as though the bridging circuit were of infinite imfrequency corresponding to wave the imk pedance of branch A as seenfr'om point C is ,infinity. Inother words, at such ajrequency the branch Ais, effectively. removed ir'qm the circuit.

The conditions given above can be readily realized physicallyso long as x is'g re'ater than A, but, obviously, cannot be realized if x is less than k. However,'the same condition of infinite impedance for the .wave A, may be, accomplished if the short circuit at F is removed, and the conductor is increased to a length This structure is shown in Fig. 2. While this conclusion is apparent by analogy with organ pipes, or

anysystem inwhich standingwaves may be set up,

it may be demonstrated mathematically in a manner similar to that useddnyonnection with Fig. 1. Such treatment would lead to the expression for the impedance of the line or network D of Fig. 2 as seen from the point of attachment -1 +-t'an-0"tan 1:.0 IS- mn In this Fig. 2 then, the impedance of the line D is infinity for the wavelength g qigt ljn the point of attachment may be, so lorigas'it is not at the mid-point, a On the other hand, by making the attachment at the distance of V ,N Z

:41;. l 1.! iii. z tfrom one, end, th stransnnssion line acts as a short-circuit, for the, wave 1e h A.

for illustrative purposes I have used the wave n line divides 5 for branch A. isi greater than "A, the

length of the line D is shown as equalto V T7 thhtfithfa made at a distance from the open end of D of I could be. open at both ends in which case its length would then be ti lyshown LIntheftreatrnentfabove, the impedance has branches, "aeaisteu, therefore, for ,two fiquen- ,cies,,,1t is. apparent that the principles of the ion ihziybeer'rthded to a larger number l on,1i could bebridged 'acr oss the line, at','appro piiate distances from the junction 1, each oi these bridged transmission lines being ofthe length and each connected at a distance from 'theopen end of one-quarter of, the wave length to be excluded. In this case, however, additional networks 1 will be require din connection with some of the bridging transmission; s jtocausethem' t have an infinite impedanceat more'than one frequency. Thus, the invention may be extended to a multiplicity of antennae. l; Various other modineations, of my invention will be apparentto those,skilled in the'iart f For instance, it will be evident that the invention is equally applicable fer the case where a single antenna is to be used at several frequencies rem g Y thefuse 'of" impedance matching devices. 'In'that' event, the networks 'or transmissionlines would be used to select automatically the proper impedance matching equipment without the intermediary of mechanical switching devices.

Again, the principles of the invention may be applied to the case where an antenna is fed at a plurality of frequencies, simultaneously, from a plurality of sources. In this case the branch lines A and B of Fig. 4 would be connected to the sources, instead of to the antennas, and the main transmission line would be connected to the antenna rather than to the single transmitter.

What is claimed is:

1. In radio transmission, a plurality of antennae, a transmission line, a branch therefrom to each antenna, and a transmission line bridged across one branch and of a length to permit the flow of the desired frequency down that branch and to exclude other waves of another frequency.

2. In radio transmission, a plurality of antennae, a transmission line, a branch therefrom to each antenna, and a transmission line bridged across one branch, the line being open at both ends and of a length equal to an integral multiple of one-half the wave length of the wave to be transmitted.

3. In radio transmission, a plurality of antennae, a transmission line, a branch therefrom to each antenna, a transmission line bridged across one branch, the line being open at both ends and of one-half the wave length of the line to be transmitted, and attached at a distance from one end of one-quarter wave length of a frequency to be excluded.

4.1m radio transmission, a plurality of antennae, a transmission line, a branch therefrom to each antenna, and a transmission line bridged across one branch, the said line being open at one end and closed at the other end and of a length equal to one-quarter of the wave length to be transmitted.

5. In radio transmission, a plurality of antennae, a transmission line, a branch therefrom to each antenna, a transmission line bridged across one branch, the said line being open at one end and closed at the other end and of a length equal to an odd multiple of one-quarter of the wave length to be transmitted, and attached at a distance from the open end of one-quarter of the wave length of a frequency to be excluded.

6. In radio signaling systems, a transmission line, two branches therefrom, one to each of two antennas, a supplemental transmission line bridged across each branch the length of the line in the branch for the longer wave length being an odd multiple of one-quarter of the wave length of the wave to be transmitted and the length of line in the other branch being an integral multiple of one-half the wave length of the wave to be transmitted by that branch, the first bridging line being closed at one end and the second bridging line being open at both ends.

7. The combination of claim 6 further characterized by the fact that the two bridging lines are connected to the branches at distances from the open end of one-quarter of the wave to be excluded.

8. The combination of claim 5 further characterized by the fact that the bridging line is connected at a distance from the junction point of the branches of one-quarter of the wave length of the frequency to be excluded.

9. The combination of claim 6 characterized by the fact that the two bridging lines are connected to the branches at distances from an open end of the bridging line, of one-quarter of the wave to be excluded, and that each bridging line is connected to its branch at a distance from the junction point of the transmission line equal to one-quarter of the Wave length of the wave to be excluded.

GEORGE W. GILMAN. 

